In recent years, hydraulic press brakes have been developed in various ways. The configuration of a conventional, general hydraulic press brake may be briefly described as follows.
A general hydraulic press brake includes a body frame, and a lower table on which a die is detachably held is provided to a lower part of this body frame. Moreover, an upper table on which a punch is detachably held is provided to an upper part of the body frame in such a way as to face the lower table in the vertical direction and to be capable of being raised and lowered (movable in the vertical direction).
A lift cylinder configured to raise and lower the upper table is provided on each of both sides, in the longitudinal direction, of the tables (the lower table and the upper table) on the body frame. Moreover, each lift cylinder includes a tubular cylinder body and a piston provided inside the cylinder body in such a way as to be capable of being raised and lowered. The inside of the cylinder body is divided vertically into an upper hydraulic chamber and a lower hydraulic chamber by the piston.
A piston pump configured to supply pressure oil to the upper hydraulic chamber and the lower hydraulic chamber of each lift cylinder is provided at an appropriate position on the body frame. Moreover, the piston pump includes a pump rotary shaft, a rotary motor configured to rotate this pump rotary shaft, and an inclined plate inclined with respect to the pump rotary shaft. The inclination angle of the inclined plate with respect to the pump rotary shaft is constant (unchanged), and the pump discharge volume is set based on this inclination angle.
Here, the speed of raising and lowering of the upper table is set to a high speed in the case where the actuation state of the lift cylinders is a no-load state, and is set to a low speed in the case where the actuation state of the lift cylinders is a high-load state.
As this type of technique, those described in literatures listed below have heretofore been known, for example (Patent Literatures Japanese Patent Application Publication Nos. Hei 7-266086 and Hei 7-275946).